186 THE BOOK OF THE LANTERN. 



self with the necessary apparatus for showing them upon 

 the screen. Or he may be employing a large lantern to 

 illustrate other parts of his lecture, which would be quite 

 unsuitable, or, at any rate, would have to be re-arranged 

 before a single spectrum experiment could be shown. 

 Feeling this want myself, I devised a plan for showing 

 spectra diagramatically with an ordinary biunial lantern. 

 (It will presently be seen that a double lantern is a neces- 

 sity for this particular manner of working), and I have 

 found the method adopted to answer admirably. A special 

 set of slides is required, but these are not at all difficult 

 to make. They must be home-made, for they are not to 

 be bought at present, although one well-known optician 

 was so pleased with the idea when I described it to him, 

 that he expressed his intention of manufacturing slides of 

 my pattern. The first of the set is a photographic slide 

 showing Newton's well-known experiment with a prism, 

 traversed by a beam of light admitted through an aperture 

 in the shutter of a darkened room. The next slide is a 

 simple-coloured band, or continuous spectrum. This is at 

 length replaced by a similar band, no longer continuous, but 

 crossed by the principal Frauenhofer lines, which are duly 

 marked above with their own distinguishing letters. Such 

 a spectrum can be copied from any work on optics, and 

 drawn and coloured on ground-glass, as explained in 

 another part of this book. We must now prepare a set of 

 slides to serve as " effects " for this last spectrum slide, 

 and which will consist of simple bright lines. The most 

 simple of these would be that due to the metal sodium, 

 which would consist of a double yellow line, to agree in 



